There are crystals in my wash buffer concentrate that won’t dissolve.

This can be rectified by either warming the wash buffer concentrate to 60°C in a water bath, or a quicker method is to decant the entire contents of the bottle into 1 litre bottle, rinse out the bottle multiple times with distilled water to collect any crystals or remaining buffer. Make to 1000mL with distilled water and mix on magnetic stirrer and the crystals will dissolve.